1. Field of the Invention
The present invention relates to a circuit for driving an organic electroluminescent (EL) circuit.
2. Description of the Related Art
An organic electroluminescent (EL) device comprising a transparent substrate on which an anode, at least organic layer, and a cathode are formed is disclosed, for example, in Japanese Patent Publication No. 32307/1994. The anode is made of a transparent conductive material. The organic layer consists either of an organic emissive layer or of a multilayer structure containing organic layers such as a hole injection layer. If necessary, a hole transport layer is added to the multilayer structure. The cathode is made of a conductive metal material.
In this organic EL device, a DC voltage of several volts to tens of volts is applied between the anode and the cathode to cause the organic layer to emit light. The organic EL device has the advantage that it can be driven at a lower driving voltage than other kinds of EL devices.
Generally, a driver circuit for this kind of organic EL device is composed of a constant-current circuit. A known circuit is shown in FIG. 7, where the organic EL device is indicated by 01 and has an anode 02 and a cathode 03. A DC power supply supply 06 such as a battery is connected with the EL device 01 via a series combination of a transistor 04 and a resistor 05. A series combination of a zener diode 07 and a resistor 08 is connected in parallel with the DC power supply 06. The neutral point of this series circuit is connected with the positive terminal of an operational amplifier 09. The negative terminal of the amplifier 09 is connected with the junction of the amplifier 09 and the transistor 04. This driver circuit limits the current supplied to the organic EL device 01 to a certain value at all times.
This driver circuit for an organic EL device needs numerous electronic components including transistor 04 and operational amplifier 09. Where numerous display segments are formed on the anode 02, or where plural anodes 02 and plural cathodes 03 intersect to form a matrix structure, it is necessary to prepare the aforementioned electronic components for each of the electrodes. This increases the number of components, leading to an increase in the cost.